Most reputable charities will remove you from their calling list if you request it - but don't invoke the "I'm on the do not call list" line, because charities are exempt from the Do Not Call list (as are political calls and market research firms). Most times, a simple "Please remove me" is all it takes. I try to keep my donations local, and that's what I tell telephone callers. It's just a waste of their time to call me to ask for money, and most have quit calling me.
As for mail solicitations, you might try sending them back and asking to be removed from their distribution list. Personally, I just toss them in the recycle bin. I don't get upset that they send them, but I think about all the money the charity is spending on paper, postage, labor to stuff the envelopes, etc. I wonder if they get enough donations back to cover their expenses!